d
(d) \(\tau = C.\theta = \frac{{\pi \eta {r^4}\theta }}{{2L}} = \)Constant

\( \Rightarrow \frac{{\pi \eta {r^4}(\theta - {\theta _0})}}{{2l}} = \frac{{\pi \eta {{(r/2)}^4}({\theta _0} - \theta ')}}{{2(l/2)}}\)

\( \Rightarrow \frac{{(\theta - {\theta _0})}}{2} = \frac{{{\theta _0}}}{{16}} \Rightarrow {\theta _0} = \frac{8}{9}\theta \)